Method for increasing the dry residue in processed cyclone dust

ABSTRACT

During processing of cyclone dust ( 4 ) resulting from the chlorination of titanium ore, a disposable filter cake ( 26 ) having a dry residue of greater than 40% (without inert portions serving as a supporting structure), regardless of whether ilmenite, slag or natural or artificial rutile or mixtures thereof were used as a raw material, is obtained when one or a combination of the following method steps is carried out: before increasing to a pH value ranging from 9 to 12 in a neutralization tank ( 13 ), a rapid neutralization step ( 12 ) is conducted whereby for all volume elements of the liquid, the same precipitation conditions can be maintained in a pH range of 6 to 9; a (preferably anionic) flocculation aid ( 22 ) is added before thickening, and; a (preferably cationic) second flocculation aid is mixed into the slurry after thickening. All measures improve the flocculation and render the processing more economical.

[0001] The invention relates to a method for processing, in a mannersuitable for landfilling, heavy-metal chlorides obtained in the form ofcyclone dust during the manufacture of titanium dioxide by the chlorideprocess, where insoluble constituents are first separated from thecyclone dust slurried with water or acids, after which the pH value ofthe solution is raised to a pH ranging from 9 to 12 by adding aneutralising agent, the heavy metals are precipitated in the form ofhydroxides, and the hydroxides are thickened in the form of sludge andfurther dewatered.

[0002] A method of this kind is known from DE 41 31 577 A1, whichdescribes that the (heavy) metal chlorides—except ironchloride—inevitably occurring during the manufacture of titanium dioxideby the chloride process have to be converted into products suitable forlandfilling and that a filter cake suitable for landfilling is obtainedif the inert constituents also contained in the cyclone dust are notseparated prior to neutralisation of the dust, thereby accepting thatthe volume for landfilling becomes substantially larger and there-usable, inert components of the cyclone dust are dispensed with. Ifthe teaching given in DE 41 31 577 A1 is applied, it is also possible toobtain a filter cake suitable for landfilling with a dry residue of lessthan 40%, usually in the region of 35%, without inert materials as afilter aid when using slag or slag/rutile blends consistingpredominantly of slag as the raw material. However, when using naturalor synthetic rutile in the chloride process, the known method ofreprocessing causes the dry residue to drop below the thixotropy limitduring sludge dewatering. A filter cake of this kind cannot be handledand is not suitable for landfilling.

[0003] The object of the invention is a method for the economical,industrial-scale reprocessing of heavy-metal chlorides occurring duringthe chlorination of titanium ore; in particular, a product suitable forlandfilling with a dry residue of more than 40% is to be obtained fromthe cyclone dust following separation of the inert constituents,regardless of the raw material used.

[0004] The object is solved in that a readily filterable floccule isproduced in the course of the process, this being achieved by one—orpreferably a combination—of the following process improvements: Rapid,homogeneous pre-neutralisation to a pH value in the range from 6 to 9 iscarried out prior to adjusting the pH value to between 9 and 12 in theneutralisation stage.

[0005] A (preferably anionic) flocculant is mixed into the suspensioncontaining the precipitated heavy-metal hydroxides prior to thickening.

[0006] A (preferably cationic) flocculant is mixed into the thickenedsludge.

[0007] It is advantageous to dewater the sludge in a membrane filterpress, the filter pressure preferably being above 4 bar and therepressing pressure above 10 bar. Each of these individual measures—andcertainly any combination—results in a filter cake suitable forlandfilling with a dry residue of over 40% by weight without inertmaterials as a filter aid, regardless of the raw material fed into thechlorinator.

[0008] The invention is illustrated in the drawing, and an example isdescribed in further detail below. The Figure shows a diagram of theprocess for reprocessing cyclone dust.

[0009] When manufacturing titanium dioxide by the chloride process, thevolatile metal chlorides (1) formed in the chlorinator are cooled tosuch an extent that, apart from titanium tetrachloride, they are allseparated in a cyclone (2) in condensed form along with the inertconstituents—mainly unreacted ore and coke particles. The titaniumtetrachloride (3), which is still gaseous at this temperature, issubsequently condensed (not shown here) and the remaining chlorinatoroff-gases are passed to a gas scrubber. The solids mixture separated inthe cyclone is referred to as cyclone dust (4).

[0010] Slurrying of the cyclone dust (4) in a tank (5) by adding waterand/or acidic process waste water (6), which occurs in the plant andneed not be expensively purified due to being used in this way, producesan acidic suspension (7): all metal chlorides are dissolved, while theinert solid particles (8), coke and unreacted ore, remain unchanged andcan be put to further use. They can easily be separated in a filter (9).

[0011] The Figure outlines a method according to the invention, by whichnone of the heavy metals present in the solution (10) are to beselectively recovered; all heavy-metal ions are to be landfilled asunserviceable material. They are converted into metal hydroxides byneutralisation. It would be perfectly possible to selectively separateindividual ions or ion groups (such as iron or vanadium) at this pointand subject them to reprocessing. Process steps of this kind, which arefamiliar in themselves, do not disrupt or alter the method according tothe invention and will therefore not be discussed further.

[0012] It has proven particularly favourable for the dewateringnecessary at the end of the process to divide neutralisation into twostages: “rapid” pre-neutralisation and subsequent adjustment. Rapidpre-neutralisation is performed in a stirred (11), small pre-mixing tank(12), this achieving a rapid and, above all, spatially and temporallyuniform rise in the pH value, thereby ensuring that, even in the eventof fluctuations in the operating cycle, no partial volumes occur inwhich nucleation and flocculation are spatially and temporallysub-optimum. The pH value increases to between 6 and 9 duringpre-neutralisation. A second (larger) neutralisation tank (13) isprovided for stabilisation and precision adjustment; the suspension (14)discharged from there has a uniform pH value of between 9 and 12,preferably in the region of 10. Neutralisation is preferably performedusing milk of lime (15) from a tank (16). A pH value controller (17) isindicated.

[0013] All heavy-metal ions from the cyclone dust are precipitated inthe form of hydroxides as a result of this neutralisation. Theintermediate connection of the stirred pre-mixing tank (12) has aninfluence on flocculation, appearing primarily to have a positive effecton the uniformity of the floccules.

[0014] Sludge (19) settles from the suspension (14) in a thickener (18)and is conveyed to a filter press (21) by a pump (20), via a tank (24)and a pump (25).

[0015] Two further measures are capable of exerting a favourableinfluence on flocculation, with an impact on filterability:

[0016] A first flocculant (22), preferably an anionic flocculant, can beadded upstream of the settling tank (18). The anionic flocculants opento consideration include, for example, copolymers of acrylamide andsodium acrylate, the preferred quantity being in the range from 5 to 30ppm, referred to the total quantity of suspension.

[0017] Also, a second flocculant (23), preferably a cationic flocculant,can be added after thickening (18). The cationic flocculants open toconsideration include, for example, cationic acrylamide copolymers, thepreferred quantity being in the range from 5 to 30 ppm, referred to thequantity of thickened sludge.

[0018] Also of influence as regards the dry residue of the filter cake(26) to be landfilled is dewatering, which is preferably performed usinga membrane filter press (21). The filter pressure should be greater than4 bar, preferably 10 to 15 bar. Repressing of the sludge is preferablyperformed at 10 to 15 bar.

[0019] Using these exemplary methods, the dry residue in the filter cakecan be increased to more than 45%. This value is achieved regardless ofthe raw material used. It has been found with the methods presented thatthe use of both natural and synthetic rutile as the ore to bechlorinated yields a non-thixotropic filter cake suitable forlandfilling. With ilmenite or with slag/rutile blends as the startingmaterial, either with or without selective reprocessing of the ironchloride, the advantage lies solely in the high dry solids content ofthe filter cake obtained during the neutralisation and reprocessing ofthe metal chlorides.

[0020] The following are the achievable dry residues and quantities forlandfilling when using pure rutile:

Method A (Prior Art)

[0021] When neutralising the original suspension without separation ofthe inert constituents, the filter cake is non-thixotropic; the dryresidue reaches 46.5% and the quantity to be landfilled is 1,115 kg permetric ton TiO₂.

Method B (Prior Art)

[0022] If the inert components are removed from the suspension in MethodA, the filter cake is thixotropic (not suitable for landfilling); thedry residue reaches only 26.9% and the quantity to be landfilled is1,270 kg per metric ton TiO₂.

Method C (Prior Art)

[0023] When treating the suspension in accordance with Method IIIdescribed in DE 41 31 577 A1, the filter cake is non-thixotropic; thedry residue is 38.6% and the quantity to be landfilled is 627 kg permetric ton TiO₂.

Method D

[0024] The method according to the invention produces a non-thixotropicfilter cake with 45% dry residue. The quantity to be landfilled is only538 kg per metric ton TiO₂.

1. Method for reprocessing, in a manner suitable for landfilling,heavy-metal chlorides obtained in the form of cyclone dust during themanufacture of titanium dioxide by the chloride process, where insolubleconstituents are first separated from the cyclone dust slurried withwater or acids, after which the pH value of the solution is raised intothe pH range from 9 to 12 by adding a neutralising agent, the heavymetals are precipitated in the form of hydroxides, and the hydroxidesare thickened in the form of sludge and further dewatered, characterisedin that the dry residue of the sludge to be landfilled is increased toover 40% by weight as a result of improved flocculation.
 2. Methodaccording to claim 1, characterised in that the improved flocculation ispromoted by rapidly increasing the pH value to pH 9 prior to precisionadjustment of the pH value, this resulting in uniform and rapidadjustment of the pH value in all volume elements of the solution. 3.Method according to claim 1, characterised in that the improvedflocculation is promoted by adding a first flocculant prior tothickening in the settling tank.
 4. Method according to claim 1,characterised in that the improved flocculation is promoted by adding asecond flocculant prior to dewatering of the sludge.
 5. Method accordingto claim 1, characterised in that the improved flocculation is promotedby combining two, or all three, of the measures according to claims 2 to4.
 6. Method according to claim 3, characterised in that an anionicflocculant is used as the first flocculant.
 7. Method according to claim4, characterised in that a cationic flocculant is used as the secondflocculant.
 8. Method according to one or more of claims 1 to 7,characterised in that the sludge is dewatered with a membrane filterpress.
 9. Method according to claim 8, characterised in that the filterpressure in the membrane filter pressure is greater than 4 bar. 10.Method according to claim 8 or 9, characterised in that the repressingpressure in the membrane filter press is greater than 10 bar.